1. Field of the Invention
The present invention relates to an optical wave guide or optical fiber made of a transparent dielectric such as glass and to be used as a transmission line for optical communication.
2. Description of the Prior Art
As a transmission line in the field of optical communications, an optical wave guide made of a fiber of such transparent dielectric as glass (usually called an optical fiber) is being developed. Important factors of a transmission line for optical communications are that the transmission loss is low, the transmission band is wide, connection is easy, maintenance is easy, and transmission characteristics are stable or they do not fluctuate due to external conditions.
An effective optical fiber having hitherto been known is the so-called clad type. This optical fiber is made up of a core portion whose optical refractive index is comparatively high, and a clad portion whose refractive index is somewhat lower than that of the core portion. Light waves propagating through the interior of the optical fiber have a mode similar to electromagnetic waves within a wave guide. Depending on the transmission mode or modes of light, the optical fiber can be classified into a single mode fiber and a multi-mode fiber.
The single mode fiber has a single transmission mode, and is free from the group delay difference between modes. It is therefore advantageous in having a wide transmission band. The transmission bandwidth is as large as several GHz at a transmission distance over 1 km. On the other hand, in order to transmit light in the single mode, it is necessary either to keep the diameter of the core at several microns or to make the difference in the refractive index between the core and the clad small. With such a small core diameter, it is extremely difficult to project light into the core of the optical fiber and to align optical fibers when they are to be connected. The small difference in the refractive index brings about the disadvantage that the emission loss due to bending of the optical fiber is great. The disadvantage causes serious bottlenecks in the practical use of the single mode fiber.
On the other hand, the multi-mode fiber obviates the foregoing disadvantages of the single mode fiber fairly well. Since, however, the many modes have differing transmission speeds, the trasmission band is limited by the group delay differences among the modes. For this reason, the important requirement for a communication line in that the light transmission line has a wide transmission band is lost. The limited bandwidth is also a great obstruction in putting optical fibers into practical use. Although a variety of countermeasures have been proposed, a practical available measure has not been realized yet.